Assuming your newest dog is a puppy I would of course recommend ensuring that the full vaccination program is complete (first and second vaccination) before letting him/her out. There should be good protection a few days after the second vaccine is administered.

Parvovirus can survive for several months and is considered ubiquitous in the environment. Exactly how long the virus stays in the environment depends on temperature, whether in direct sunlight, level of humidity and so on. After several years I would think the yard is of low risk. Was it disinfected after your last dog's illness?

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